An organic electroluminescence (EL) device includes a fluorescent organic EL device or a phosphorescent organic EL device, and a device design optimum for the emission mechanism of each type of organic EL device has been studied. It is known that a highly efficient phosphorescent organic EL device cannot be obtained by merely applying fluorescent device technology due to the emission characteristics. The reasons therefor are generally considered to be as follows.
Specifically, since phosphorescence emission utilizes triplet excitons, a compound used for forming an emitting layer must have a large energy gap. This is because the energy gap (hereinafter often referred to as “singlet energy”) of a compound is normally larger than the triplet energy (in the invention, the difference in energy between the lowest excited triplet state and the ground state) of the compound.
In order to confine the triplet energy of a phosphorescent dopant material efficiently in an emitting layer, it is required to use, in an emitting layer, a host material having a triplet energy larger than that of the phosphorescent dopant material. Further, an electron-transporting layer and a hole-transporting layer are required to be provided adjacent to the emitting layer, and a compound having a triplet energy larger than that of a phosphorescent dopant material is required to be used in an electron-transporting layer and a hole-transporting layer.
As mentioned above, if based on the conventional design concept of an organic EL device, it leads to the use of a compound having a larger energy gap as compared with a compound used in a fluorescent organic EL device in a phosphorescent organic EL device. As a result, the driving voltage of the entire organic EL device is increased.
Further, a hydrocarbon-based compound having a high resistance to oxidation or reduction, which has been useful in a fluorescent device, the π electron cloud spreads largely, and hence it has a small energy gap. Therefore, in a phosphorescent organic EL device, such a hydrocarbon-based compound is hardly selected. As a result, an organic compound including a hetero atom such as oxygen and nitrogen is selected, and hence a phosphorescent organic EL device has a problem that it has a short lifetime as compared with a fluorescent organic EL device.
In addition, a significantly long exciton relaxation speed of a triplet exciton of a phosphorescent dopant as compared with that of a singlet exciton greatly effects the device performance. That is, emission from the singlet exciton has a high relaxation speed that leads to emission, and hence, diffusion of excitons to peripheral layers (such as a hole-transporting layer and an electron-transporting layer) hardly occurs, whereby efficient emission is expected. On the other hand, in the case of emission from the triplet exciton, since it is spin-forbidden and has a slow relaxation speed, diffusion of excitons to peripheral layers tends to occur easily, and as a result, thermal energy deactivation occurs from other compounds than a specific phosphorescent emitting compound. That is, in a phosphorescent organic EL device, control of a recombination region of electrons and holes is more important than that of a fluorescent organic EL device.
For the reasons mentioned above, in order to improve the performance of a phosphorescent organic EL device, material selection and device design that are different from a fluorescent organic EL device have come to be required.
In particular, in the case of a phosphorescent organic EL device that emits blue color light, as compared with a phosphorescent organic EL device that emits green to red color light, it is preferable to use a compound having a high triplet energy in an emitting layer or peripheral layers thereof. In order to obtain a compound that not only has a high triplet energy but also has properties required for an organic EL device, it is required to conduct molecular design based on a new concept taking the electron state of π electrons into consideration, not to simply combine molecular parts having a high triplet energy such as a heterocyclic compound.
Patent Document 1 discloses a compound having at least two rings of dibenzothiophene and having an aromatic ring or a heterocyclic aromatic ring as a linker part of the two dibenzothiophenes. Further, this document shows that a device using a carbazole-containing compound as a host compound in this linker part has a long luminous life.
Patent Document 2 discloses a compound having four or more aromatic rings or heterocyclic aromatic rings, in which one of these rings is dibenzofurane or dibenzothiophene. This document shows that a device using a compound having carbazole as an aromatic heterocyclic ring as a host compound or an electron-transporting material is excellent in luminous efficiency and luminous life.    Patent Document 1: WO2010/004877    Patent Document 2: JP-A-2011-084531